1. Field of the Invention
The present invention is directed to a process for the continuous arrangement of alkali metal salts of aromatic carboxylic acids in the presence of an organic diluent. In particular, it concerns a process for the dismutation of potassium benzoate.
2. Description of the Prior Art
High temperature rearrangement of the alkali metal salts of aromatic carboxylic acids into different salts of aromatic carboxylic acids having at least two carboxyl groups in the molecule is well documented in technical literature. The rearrangement reaction can be a dismutation reaction, producing an aromatic polycarboxylate containing at least one additional carboxyl group. Additionally, the rearrangement reaction can be an isomerization reaction, during which the number of the carboxyl groups attached to the same molecule remains unchanged, but in which the position of these various groups changes.
The method of rearranging alkali metal salts of aromatic mono-or polycarboxylic acids, commonly referred to as the Henkel process, was first performed by heating the salts in the solid state in the presence of a protective gas such as nitrogen or carbon dioxide. Unfortunately, this reaction in the solid or partially molten state causes tremendous technical difficulties when performed on the industrial scale. These difficulties arise from the low heat transfer in the reactor as well as the problems associated with transportation and mixing of both the reactants and the reaction products. Moreover, the solids have a tendency to agglomerate and form dross, which increases handling difficulty.
Consequently, it was proposed to perform the rearrangement reaction so that an inert diluent is used to transport the solids in the form of a suspension. This technique was described in French Pat. No. 1,105,556 and British Pat. No. 810,893. In these cases the processes were performed discontinuously and the recommended diluent was benzene or naphthalene, biphenyl, diphenylether, or pyridine.
The rearrangement method for alkali metal salts of aromatic carboxylic acids employing suspensions in an inert diluent was subsequently improved by the selection of more suitable diluents. Thus, it was recommended in French Pat. No. 2,143,401 to use as the diluent, aromatic hydrocarbons having at least three aromatic rings such as the aromatic polynucleic products. Examples of these dispersion products are the terphenyls, tetraphenyls, pentaphenyls, binaphytyls, anthracene, phenanthrene, pyrene, triphenylene, chrysene, perylene, and pentacene. As set forth in French Pat. No. 2,143,401, the use of these diluents permits one to perform the rearrangement reaction at very high conversion rates, resulting in a higher yield of the final product. Although the process described by the patent is performed discontinuously, there is an indication, without a description, that a continuous operation could be performed in a manner such that the suspension of the alkali metal salts of the aromatic carboxylic acids are moved by pumps. The possibility of performing the rearrangement reaction continuously is also suggested and generally described in French Pat. No. 2,213,266.
Unfortunately, the continuous rearrangement processes performed in the presence of a diluent as described in the prior art give much poorer results than those provided by discontinuous operations. In particular, for the same conversion rate, the yields of the continuous operation are much lower than those of the discontinuous operation. Thus, there remains a need in the art for a continuous process for rearranging alkali metal salts of aromatic carboxylic acids which provides yields as high as the discontinuous processes known to those of skill in the art.